A physically based raindrop–cloud droplet accretion parametrization for use in bulk microphysics schemes

Abstract

AbstractThe accretion process in bulk cloud microphysics schemes can be parametrized using the stochastic collection equation (SCE). In this study, the collection efficiency of each raindrop–cloud droplet pair is applied to the SCE to derive a new accretion parametrization that considers a strong variability of accretion rate depending on the cloud droplet and raindrop size distributions. To evaluate the new accretion parametrization (NP), it is implemented into a cloud‐resolving model, replacing the original accretion parametrization (OP) based on the continuous collection equation. In the idealized simulations of deep convective clouds, NP predicts overall larger accretion rates and smaller autoconversion rates than OP. The resultant high accretion/autoconversion rate ratio in NP increases the mean raindrop size. This induces faster sedimentation of raindrops that is associated with the earlier onset of surface precipitation and also weakens the evaporation cooling of raindrops that can affect the thermodynamics and dynamics of clouds. Meanwhile, for a given pair of rainwater and cloud water mass contents, the accretion rates in NP have a broad distribution while those in OP are less variant, suggesting that the dependence of the accretion rates on bulk microphysical properties other than the mass contents also needs to be properly considered in accretion parametrizations. Most of the aforementioned differences between the two accretion parametrizations found in the idealized simulations are also found in the real‐case simulations of a precipitation event over Bangladesh but the differences are smaller, and the spatial distribution of the accumulated precipitation amount is relatively well predicted by NP compared to OP.